The present invention relates to a hybrid circuit and method for use in a digital switching system.
With the advent of digital switches for replacement of metallic cross-point analog switches in local telephone networks, there is a need for more accurate balancing of the hybrid circuit used for two-wire to four-wire conversion. This need arises because the four-wire local switch, which was formerly two-wire, would ideally have zero dB insertion loss, as does the switch it replaces, whereas previously four-wire transmission in the telephone network always had an associated non-zero loss, improving the singing margin.
Singing in hybrid circuits is defined as an undesired self-sustaining oscillation at a frequency in the passband of the system. Singing margin is defined as the net loss around a four-wire singing path at any frequency, or the minimum value of such loss over a range of frequencies.
One prior art approach has been to provide a switch insertion loss, usually about two dB, which adversely affects lossnoise grade of service on longer subscriber loops. Another approach is to replace the former single compromise hybrid termination with two or more terminations tailored to each type of subscriber loop design. In such an approach, two such terminations, one for loaded loops and another for unloaded loops, will result in acceptable singing margin with a zero-loss switch. Approaches to choosing these terminations have included manual switch setting from plant records and automatic setting by transmitting a tone and measuring the return loss during call set-up.
Loop segregation with manually switched terminations has an associated administrative expense, which would not be well received by telephone operating companies. Problems with transmitting a tone and measuring return loss are that the tone transmitted with the phone off-hook may annoy the customer, intervention by the switch common control and communication between the switch common control and line interface may be necessary, and continued adaptation during the course of the call is not possible but would, however, be desirable because of extensions, foreign equipment and change of line impedance, among others.
In addition, due to resonances in the impedance characteristic, measurements of impedance at a single frequency may be misleading.
Another approach is an adaptive echo canceller, which is of primary interest in satellite transmission, in that it adapts on the basis of virtually any input signal, including speech. A problem with adaptive echo cancellers is circuit complexity.
In view of the above background, it is an objective of the present invention to provide an improved hybrid circuit for use in a digital switching system.